1. Field of the Invention
The present invention relates to patterning substrates, and more particularly to method of patterning an underlying substrate using spin-on glass as a hard mask.
2. Description of Related Art
Integrated circuit manufacture requires that precisely defined regions be exposed to etchants in order to selectively remove material from the regions. The patterns that define the regions are often created by lithographic processes. For instance, a layer of photoresist is spin-coated onto a wafer substrate, and the resist layer is selectively exposed to a form of radiation, such as ultraviolet light, electrons, or x-rays. An exposure tool and mask, or data tape in electron beam lithography, are used to effect the desired selective exposure. Openings in the resist corresponding to the selective exposure are formed in a subsequent development step. Next, an etch is applied, and the regions of the substrate exposed by the openings are removed. The etching provides subtractive pattern transfer to the underlying substrate.
The photoresist performs two primary functions: precise pattern formation, and protection of the substrate during etch. Photoresist resolution refers to the ability of the photoresist to accurately match the pattern of the lithographic exposure equipment. Photoresist resolution is a key parameter that depends on several factors including the contrast, swelling, and thickness of the photoresist, as well as proximity effects (e.g., an isolated opening). As the photoresist becomes thicker, the openings near the bottom surface tend to narrow. As a result, thicker photoresist tends to limit the resolution. Therefore, generally speaking, the resolution is improved by thinning the photoresist. That is, thinner resist allows extending standard exposure tools to finer geometries. However, the photoresist must remain thick enough to avoid being removed by the etchant. For instance, for submicron patterns, the photoresist might be approximately 2 to 3 times as thick as an underlying metal substrate in order to adequately protect the substrate from an etchant that is particularly harsh to the photoresist. Unfortunately, at this thickness, the resolution of the photoresist may produce significant deviations between the desired pattern and the actual pattern transferred to the substrate. Accordingly, there is a need for using thinner photoresist layers while assuring that covered portions of the substrate remain protected from the etch.
Spin-on glass is finding increasing use as an interlevel insulator that can be planarized. Spin-on glass is of particular interest because the deposition process and planarization is relatively simple and the process utilizes low cost equipment.
U.S. Pat. No. 5,264,076 to Cuthbert et al. discloses an integrated circuit process using a non-conformal layer of spin-on glass as a hard mask for patterning an underlying nonplanar layer of polysilicon. The method includes forming nonplanar features on a substrate, covering the features with a conformal layer of polysilicon, covering the polysilicon with a non-conformal layer of spin-on glass, and covering the spin-on glass with photoresist. The spin-on glass provides a planar surface for the resist in order to reduce variations in resist thickness that cause variations in linewidths. The photoresist is patterned using lithography, and then the spin-on glass is patterned using the photoresist as an etch mask. Next, the photoresist is removed. Thereafter, the underlying polysilicon is etched using the spin-on glass as a hard mask. A drawback to this approach is that a separate step is required for removing the photoresist, which adds unnecessary process complexity. Another drawback is that the photoresist does not assist the spin-on glass when the polysilicon is etched.
Accordingly, a need exists for an efficient method of patterning a substrate using a thin layer of photoresist while adequately protecting the substrate from the etch.